1. Field of the Invention
This invention relates to an eccentric orbiting type planetary gear device, and a method of manufacturing the device, and more particularly to an eccentric orbiting type planetary gear device in combination with bearings, and a method of manufacturing the device.
2. Description of the Related Art Japanese Patent Application (OPI) No. 41240/1991 has disclosed the following eccentric orbiting type planetary gear device: That is, a supporting member and a disk, which form a supporting block, are machined in advance and separately so that they have rolling surfaces corresponding to inner races. The supporting member and the disk thus machined are joined together by tightening bolts with an external gear interposed in the supporting member and the disk. The amount of movement of the supporting member and the disk which is due to the tightening force is utilized to apply a predetermined pre-load to bearings interposed between the supporting block and an internal gear member.
As was described above, in the conventional eccentric orbiting type planetary gear device in which the external gear is interposed between the supporting member and the disk, the supporting member and the disk are machined individually, and then assembled. Each of those components has its own machining error. Hence, when the components are assembled, the errors are added, providing a relatively large error value in total. Hence, it is essential to control the pre-load applied to the bearings inserted between the supporting block and the body.
A variety of pre-load control methods are known in the art. In one of the methods, a distance between bearings is measured, or a gap in an axial direction is measured. In another method, as was described with reference to Japanese Patent Application (OPI) No. 41240/1991, the pre-load is applied by tightening the bolts. In another method, a distance piece is set between each pair of bearings, or as shown in FIG. 16 a distance piece is set at the end of the bearing, thereby to adjust the gap between bearings in the axial direction.
If this pre-load adjustment is unsatisfactory, then rotational torque variation occurs, which may results in lost motion when the direction of rotation is reversed, or may give rise to a problem that rigidity against bending moment is lowered. Hence, if this eccentric orbiting type planetary gear device is employed for a positioning device, the positioning accuracy is unavoidably unsatisfactory.
Furthermore, the above-described conventional eccentric orbiting type planetary gear device is disadvantageous in that it is made up of a large number of components. Especially, in the case of the device using the distance pieces, it is necessary to prepare several kinds of distance pieces different in width. On the other hand, a method may be employed in which bearings high in accuracy are used to reduce the amount of adjustment. However, the method involves another problem that, since a bearing comprises an inner race, an outer race, rolling elements such as rollers, and a holder, the employment of a bearing high in accuracy will increase the manufacturing cost according to the degree of accuracy of the bearing.
Moreover, in the conventional eccentric orbiting type planetary gear device, the gap between the bearing and the bearing inserting section as viewed in the radial direction affects the lost motion of the device. In addition, the gap is not uniform in radial directions. This fact results in variations in the performance (the rigidity against bending moment), thus adversely affecting the performance of the device.
Heretofore, in order to reduce the radial gap, the bearing including the inner and outer races is press-fitted in the bearing inserting section. To do so, a great force is required; that is, it is necessary to use a press-fitting device. On the other hand, it takes time and labor to join the bearing and the bearing inserting section, which results in an increase in manufacturing cost. In addition, the pre-load adjustment is necessary during assembling. That is, the conventional eccentric orbiting type planetary gear device is low in assembling efficiency.